The most important conclusion from a comparison of how bacteria and higher forms of life obtain energy is that all living creatures have a common fermentative metabolism, which suggests a common evolutionary origin. The various types of respiration and photo- or chemosynthesis that have been added to glycolysis, do not obscure this basic unity.

Oxidation with O2, yields vastly more energy than fermentation alone, and had oxygen been present at the time when life evolved, it surely would have been used. In that case, O2 respiration would be as common to all life as fermentation is.

However, this is not the case, which leads us to a second conclusion: Life arose from less complex, nonliving chemical systems at a time when the atmosphere was reducing in character, not oxidizing.

Other evidence points to the same conclusion.

The atmospheres of the other planets generally are reducing, as we shall see later in the section on geological evidence.

Old mineral beds on this planet suggest that they were laid down in contact with a reducing atmosphere.

Organic compounds themselves are unstable in an O2 atmosphere, and are auto-oxidizable. Organic matter today is constantly produced anew by the action of living organisms. If all life were to end tomorrow, O2 would begin to reclaim the organic matter on our planet, and the process would stop only when no more free oxygen remained.

It is inconceivable that large quantities of organic substances could have remained unoxidized long enough for life to evolve from them, if they were constantly exposed to 02 in the atmosphere.

If the original atmosphere was reducing, why is it oxidizing today?

One source of O2, is the photodissociation of water vapor by ultraviolet light in the upper atmosphere, followed by the loss of light hydrogen atoms from the Earth's gravitational field. This alone could lead to an oxygen concentration of around 0.1% of the present-day level.

The main source of oxygen in the atmosphere today is green-plant photosynthesis, and this probably is what turned the planetary atmosphere from reducing to oxidizing.

Life evolved under reducing conditions, where organic molecules would be stable for long periods of time; but this same life was responsible later for changing the original atmosphere to its present-day composition.